Add comment to clarify the use of `str'

1 files changed, 2 insertions, 0 deletions

diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index d1fa5c7f120..ddfa7a3e15c 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -5443,6 +5443,8 @@ This returns non-nil only if we actually send anything."
     (let ((str input)
           (erc-insert-this t))
       (setq erc-send-this t)
+      ;; The calling convention of `erc-send-pre-hook' is that it
+      ;; should change the dynamic variable `str'.
       (run-hook-with-args 'erc-send-pre-hook input)
       (when erc-send-this
         (if (or (string-match "\n" str)